body {
  display: flex;
  justify-content: center;
  align-items: center;
  height: 100vh;
}

*,
*::before,
*::after {
  box-sizing: border-box;
}

@keyframes rotate {
  100% {
    transform: rotate(1turn);
  }
}

.rainbow {
  position: relative;
  z-index: 0;
  width: 400px;
  height: 300px;
  border-radius: 10px;
  overflow: hidden;
  padding: 2rem;

  &::before {
    content: "";
    position: absolute;
    z-index: -2;
    left: -50%;
    top: -50%;
    width: 200%;
    height: 200%;
    background-color: #399953;
    background-repeat: no-repeat;
    background-size: 50% 50%, 50% 50%;
    background-position: 0 0, 100% 0, 100% 100%, 0 100%;
    background-image: linear-gradient(#399953, #399953), 
      linear-gradient(#fbb300, #fbb300),
      linear-gradient(#d53e33, #d53e33), 
      linear-gradient(#377af5, #377af5);
    animation: rotate 4s linear infinite;
  }

  // 遮罩层，比 div.rainbow 小一点，留出了边框的部分
  &::after {
    content: "";
    position: absolute;
    z-index: -1;
    left: 6px;
    top: 6px;
    width: calc(100% - 12px);
    height: calc(100% - 12px);
    background: white;
    border-radius: 5px;
    animation: opacityChange 3s infinite alternate;
  }
}

@keyframes opacityChange {
  50% {
    opacity: 1;
  }
  100% {
    opacity: 0.5;
  }
}
